Cylindrical parabolic collectors or concentrators concentrate solar radiation for the purpose of attaining the high temperatures required to produce electric power.
A cylindrical parabolic collector consists of a cylindrical parabolic mirror that reflects all the solar radiation it receives on a glass tube placed along the focal line of the mirror; on the inside of the latter is an absorbent tube through which the heat carrying fluid circulates.
In order that, at all times, the result of the reflection on our mirror is that the solar radiation strikes the absorbent tube, our cylindrical parabolic collector must follow the sun, thus requiring a solar tracker device, i.e., a tracking device of the sun's position that rotates the cylindrical parabolic mirrors of the collector around a shaft, such that the solar rays always strike perpendicularly to the collector opening. Therefore, it is essential that the measurement of the angular position of the collector structure be as accurate as possible.
The angular position of the shaft that rotates together with the collector structure should be measured in the centre of this shaft, since a measurement at the ends would not be optimal because of the torsion of this shaft. It is therefore not advisable to place a conventional encoder on one end of this shaft to obtain a reliable measurement.
To be able to measure the angular position of the structure shaft, there are encoders available that consist of a magnetized tape at defined distances and a sensor head. When a relative movement occurs between the sensor head and the magnetized tape, the head generates a series of electric pulses that can be read by a programmable logic controller to calculate the angular position of the collector.
The normal practice to secure the magnetized tape to the structure's shaft of rotation consists of sticking it directly on this shaft by the non-magnetized surface, such that the magnetized surface is facing out so that it can be read by the sensor head. However, one drawback of this arrangement is that securing the tape by sticking it on is not durable, which means that the magnetic tape eventually comes unstuck at its ends. Moreover, as it is a permanent fastening system, if any error is made during the first installation of the tape, it is very difficult to rectify it later because, in the event that the tape comes unstuck, it is not possible to again stick it onto the surface it was on. This surface also has a certain roughness that hinders the sticking process.